The present invention relates to a corona charging device for depositing charge on an adjacent surface. More particularly, it is directed to a corona charging arrangement usable in a xerographic reproduction system for generating a flow of ions onto an adjacent imaging surface for altering or changing the electrostatic charge thereon. Still more particularly, this invention is directed to an improved configuration for a corona discharge device of the type disclosed in Patent Application Ser. No. 596,656, filed July 4, 1975, in the joint names of T. Davis and G. Safford, and commonly assigned.
In the electrophotographic reproducing arts, it is necessary to deposit a uniform electrostatic charge on an imaging surface, which charge is subsequently selectively dissipated by exposure to an information containing optical image to form an electrostatic latent image. The electrostatic latent image may then be developed and the developed image transferred to a support surface to form a final copy of the original document.
In addition to precharging the imaging surface of a xerographic system prior to exposure, corona devices are used to perform a variety of other functions in the xerographic process. For example, corona devices aid in the transfer of an electrostatic toner image from a reusable photoreceptor to a transfer member, the tacking and detacking of paper to the imaging member, the conditioning of the imaging surface prior, during, and after the deposition of toner thereon to improve the quality of the xerographic copy produced thereby. Both d.c. (d.c. potential connected to the coronode) and a.c. (a.c. potential connected to the coronode) type corona devices are used to perform many of the above functions.
The conventional form of corona discharge device for use in reproduction system of the above type is shown generally in U.S. Pat. No. 2,836,725 in which a conductive corona electrode in the form of an elongated wire is connected to a corona generating d.c. voltage. The wire is partially surrounded by a conductive shield which is usually electrically grounded. The surface to be charged is spaced from the wire on the side opposite the shield and is mounted on a grounded substrate. Alternately, a corona device of the above type may be biased in a manner taught in U.S. Pat. No. 2,879,395 wherein an a.c. corona generating potential is applied to the conductive wire electrode and a d.c. potential is applied to the conductive shield partially surrounding the electrode to regulate the flow of ions from the electrode to the surface to be charged. Other biasing arrangements are known in the prior art and will not be discussed in great detail herein.
Several problems have been historically associated with such corona devices. One major problem has been their inability to deposit a relatively uniform negative charge on an imaging surface. Another problem has been the growth of chemical compounds on the coronode which eventually degrades the operation of the corona device. Yet another problem has been the degradation in charging output resulting from toner accumulations on the coronode and surrounding shield structure. One still further problem is wire vibration which leads to arcing and wire fracture. These problems, among others, are specifically addressed in the aforementioned application in which there is proposed a novel corona discharge configuration which substantially reduces or alleviates the problems noted above, and other problems associated with prior art corona devices, as is discussed more fully therein.
By way of summary, the aforementioned application discloses a novel corona device for use in electrostatic reproduction machines which comprise a corona discharge wire coated with a relatively thick dielectric coating, the thickness of the coating being sufficient to prevent the flow of conduction current from the wire. Generation of charge is accomplished by means of a voltage at the dielectric surface established by capacitative coupling through the dielectric material. The magnitude of the flow of charge to the surface to be charges is regulated by the application of a d.c. bias potential to a conductive shield adjacent or contigous to the electrode.
While the above-noted corona device disclosed in Ser. No. 595,656 solves many problems associated with other known corona devices, it is desireable to provide a corona device which operates to produce higher charging currents for given operating potentials. Higher current levels in prior art devices are usually obtained by raising the operating voltages of the corona devices. As is well known in the art, corona devices when operated at relatively high potentials generate a greater amount of ozone, which may become a health hazard, if not properly controlled. Thus, higher operating voltage levels tend to produce higher ozone levels. For this reason, it would be an advantage to produce a corona device which provides a given charging current at lower energizing potential than possible with prior art devices. In addition, however, lower energizing potentials are an advantage in themselves by simplifying and reducing the cost and complexity of power supplies, insulation, etc.
A further disadvantage of conventional prior art corona discharge devices (which problem is shared by the improved corona device of application Ser. No. 595,656) results from the fact that the corona electrode or wire of such devices is commonly suspended between dielectric support blocks at the opposite ends of the device. This has the first disadvantage of setting a lower limit on the diameter of the electrode since it must have sufficient tensile strength to be supported in taut condition, and to remain in the same relative position over varying operating conditions. Expansion coefficients are also of obvious concern in selecting a suitable electrode for such prior art corona devices. Furthermore, an electrode suspended in the above manner tends to vibrate due to the high electric fields in which it is suspended. Another disadvantage resulting from the suspension of the coronode in a taut condition between supports blocks is that the wire itself is difficult to clean by abrasion.
A further disadvantage of known corona devices is that they are relatively bulky. This is due firstly to the unused space required between the coronode and the surrounding shield structure and secondly to the shield structure itself, which generally has a U-shaped cross section to partially enclose the coronode.